Not in a million years could I see Bill Murray agreeing to be in a movie with a CG Harold Ramis.

Murray and Ramis barely even talked to each other for twenty years after a falling out they had during Groundhog Day. Maybe that's the real reason Murray wouldn't do Ghostbusters 3, because he refused to work with live-action Ramis. Maybe he was just waiting Ramis out, and now that Ramis is gone, they can start work on GB3 with a CGI Egon, or recast the role, or just write the character out entirely. They have several scenarios to work with, and on the bright side, none of them can possibly be as bad as Ghostbusters 2.

Just read something about how Ramis was barely going to be in GBIII anyway and how it's still a go. Not sure if it's true, but it makes sense if it's more of a 21 Jump Street style reboot than a sequel.